I. Field of the Invention
The present invention relates generally to satellite and other communication systems subject to severe path loss, and more specifically, to a method of providing paging signals, referred to as deep paging, which results in paging signals that are receivable in the presence of high levels of attenuation.
II. Related Art
Conventional satellite-based communication systems include gateways, user terminals, and one or more satellites to relay communication signals between the gateways and the user terminals. A gateway is an earth station having an antenna for transmitting signals to and receiving signals from satellites. A gateway provides communication links, using satellites, for connecting a user terminal to other user terminals or users of other communication systems, such as a public switched telephone network. A user terminal is a wireless communication device such as, but not limited to, a cellular or satellite telephone, a data transceiver, and a paging receiver. A user terminal can be fixed, portable, or mobile, such as a mobile telephone. A satellite is an orbiting receiver, repeater, and regenerator used to relay information.
A satellite can receive signals from and transmit signals to a user terminal provided the user terminal is within the “footprint” of the satellite. The footprint of a satellite is the geographic region on the surface of the Earth within the range of signals of the satellite. The footprint is usually geographically divided into “beams,” through the use of beam-forming antennas. Each beam covers a particular geographic region within the footprint. Beams may be directed so that more than one beam from the same satellite covers the same specific geographic region.
Some satellite communications systems employ code division multiple access (CDMA) spread-spectrum signals, as disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled “Spread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeaters,” and U.S. Pat. No. 5,691,974, which issued Nov. 25, 1997, entitled “Method and Apparatus for Using Full Spectrum Transmitted Power in a Spread Spectrum Communication System for Tracking Individual Recipient Phase Time and Energy,” both of which are assigned to the assignee of the present invention, and are incorporated herein by reference.
In satellite communication systems employing CDMA, separate communication links are used to transmit communication signals, such as data or traffic, to and from a gateway. Specifically, communication signals originating at the gateway are transmitted to a user terminal over a “forward communication link,” whereas, communication signals originating at a user terminal are transmitted to the gateway over a “reverse communication link.”
On the forward communication link, information is transmitted from a gateway to a user terminal over one or more beams. These beams often comprise a number of so-called subbeams (also referred to as frequency division multiple access (FDMA) channels, or in the case of spread spectrum CDMA channels) covering a common geographic area, each occupying a different frequency band. More specifically, in a conventional spread-spectrum communication system, one or more preselected pseudorandom noise (PN) code sequences are used to modulate or “spread” user information signals over a predetermined spectral band prior to modulation onto a carrier signal for transmission as communication signals. PN spreading is a method of spreadspectrum transmission that is well known in the art, and produces a communication signal with a bandwidth much greater than that of the data signal. On the forward link, PN spreading codes or binary sequences are used to discriminate between signals transmitted by different gateways or over different beams, as well as between multipath signals. These codes are often shared by all communication signals within a given subbeam.
In a conventional CDMA spread-spectrum communication system, “channelizing” codes are used to form multiple channels within a satellite sub-beam on a forward link. The channelizing codes are unique ‘covering’ or ‘channelizing’ orthogonal codes that create orthogonal channels in a subbeam over which communication signals are transferred. Walsh functions are generally used to implement the channelizing codes, also known as Walsh codes or Walsh sequences, and create what are known as Walsh channels. A typical orthogonal code length is 64 code chips for terrestrial systems and 128 code chips for satellite systems.
A majority of the orthogonal channels are traffic channels that provide messaging between a user terminal and a gateway. The remaining channels often include a pilot channel, a sync channel, and one or more paging channels. Signals sent over the traffic channels are generally intended for reception by one user terminal, although messages can also be broadcast to multiple users. In contrast, paging, sync, and pilot channels are generally monitored by multiple user terminals.
When a user terminal is not involved in a communications session (that is, the user terminal is not receiving or transmitting traffic signals), the gateway can convey information to the user terminal by transmitting a page to the user terminal. The page, which is usually a short message, is transmitted over the above mentioned paging channel. Pages are often sent by the gateway to establish a communication link with a user terminal, to notify a user terminal that it is being called, to reply to a user terminal trying to access the system, and for user terminal registration. Pages are also used to distribute traffic channel assignments and system overhead information to user terminals. Pages transmitted over the paging channel typically have a data rate on the order of 9600 or 4800 bits per second.
Unfortunately, a user terminal typically encounters problems receiving pages when the user terminal is inside a building or there is some structure or other obstruction positioned between the user terminal and the satellite (such as a tree, geological object, or a building). In such a situation, the user terminal is unable to acquire a page, paging message, or paging signal because the page is unable to penetrate the building or other material due to a propagation loss of the signal as it propagates through the structure. An obvious solution to overcome the propagation loss is to increase the power of the paging channel. The problem with this approach is that in order to overcome such signal attenuation or blockage the power of the paging channel has to be increased significantly. Generally, this requires increasing the signal strength to such a level that a power flux density (PFD) limit would be exceeded in the surrounding area. That is, governmental licensing restrictions and technical constraints place limits as to the allowed amount of power flux density satellite signals can have over a given area or geographical region. Increasing the power to successfully page a blocked or impeded user terminal, places the surrounding area over the allowed power flux density (PFD).
What is needed, therefore, is a method that can provide what will be called “deep paging” without increasing the power of the paging signal. Deep paging refers to being able to page a user terminal in an environment where there is an excess propagation loss to be overcome, a loss much higher than normally encountered, which is typically on the order of 20 or 30 dB. Such an environment includes a situation where a user terminal is located deeply inside a building or structure, or behind a partial obstruction.